Pulsed electric discharges are well known sources of short-wavelength light, having applications in regions of the electromagnetic spectrum from the ultra-violet (UV, wavelength λ˜300 nm) to the x-ray range (λ<1 nm). However, there is a need for stable, long-life light sources in the EUV region of the spectrum, λ=10 to 50 nm), particularly for EUV lithography (EUVL).
EUVL is expected to succeed Deep UV lithography technology for the production of silicon-based computer chips, at and beyond the 35 nm node. This technology is expected to take over fabrication in the 2007-2009 time frame. The stepper machines that print these chips are expected to cost $20-40 M each, and, in this time frame, anticipated sales of 200-300 units/year are expected, providing the three major stepper manufacturing companies, ASML (Netherlands & USA), Nikon and Canon (Japan), with a new $100 B/year market. The light sources for these steppers, are currently required to provide greater than 100 W of ‘clean power’ and can account for up to 20% of this total market. A source of sufficient power is identified as the principal problem area in the ITRS (SEMATECH) Roadmap for the development of EUVL. The roadmap has been modified periodically over the years to take into account the required increase in wafer throughput, larger (300 mm) wafers, and higher Cost of Ownership (CoO), and the power of the source demanded has progressively increased. λ Currently the total required emitted power within a solid angle of 2λ, from a source of <1 mm in size within a 2% bandwidth at a wavelength of 13.5 nm, is 400 to 1000 W. This large amount of power is the major challenge for companies developing the light sources.
There are two primary types of light sources being developed, those that depend on electrical discharge plasma, and those that use a laser-plasma source. Both approaches operates at frequencies in excess of 6 kHz, with pulse-to-pulse stability of approximately 1% . They are also required to be capable of long term operation (up time >95%), and ‘clean’ operation. By ‘clean’ operation we mean ‘debris-free’ or protected from the effects of particulate emission and plasma ions emanating from the source.
Both laser plasmas and discharge plasmas can produce high velocity particulate emission or ‘projectiles’ that will damage the expensive, precision-coated EUV collection mirrors that are in direct line-of-sight of the source. In laser plasmas, this particulate debris can originate from solid target sources, or close-proximity nozzles used to inject gaseous targets. In discharge sources the debris originates from the electrodes or from insulative materials close by. The plasma ions are, of coarse, inherent to the plasmas themselves. They need to be stopped from sputtering (ablating) the collection mirrors. Several techniques have been devised to stop the sputtering, including Repeller Field approach disclosed in U.S. Pat. No. 6,377,651 issued to Richardson, et al. on Apr. 23, 2002, which is incorporated by reference.
Companies developing discharge plasmas (DP) include Philips (Hollow-Cathode Plasma Discharge), Xtreme Technologies (HC Z-pinch), Cymer (Dense Plasma Focus), Plex LLC (star discharge), Gygaphoton (capillary discharge pinch plasma). Most of these companies are focusing their R&D activities on Xenon-based plasmas. Although the use of Xenon mitigates the debris problem to some extent, the principal drawback is its low conversion efficiency into in-band, 13.5 nm EUV light. Both DP and LP sources have been limited to conversion efficiencies (CE) of 0.5 to 0.7%. The highest known CE is 0.95%. Moreover, there are now solid, atomic physics, reason to believe that the CE of Xenon will not improve much beyond these values.
These low CE's have adverse implications for both discharge plasma and laser plasma sources. For the laser plasma it means the use of a laser system having a power in excess 40 k W, beyond known technical capabilities and possibly prohibitively expensive. For discharge plasma sources, the low CE poses extreme problems with heat removal from the source and very large electrical power requirements, approaching 1 MW.
One approach for laser plasma sources uses microscopic, mass-limited, spherical targets composed of several materials including a small amount of tin. Tin is a metal and can, in principal pose a more serious debris problem as an EUV source. However, it has the advantage that much higher CE's are possible. CE's of 1-2% have been demonstrated and there is reason to believe higher values are possible.
The possible advantages of introducing tin into the discharge region of the source have been recognized and cursory tests completed. Use of electrodes made of tin-containing material, or using some method (thermal evaporation, or electron-beam heating) to introduce a tin vapor into the discharge has been disclosed. It is believed that the results have been disappointing for one or more reasons including, creation of large amounts of debris, instabilities in the discharge, and difficulties foreseen in scaling to the required powers. These difficulties originate from the inability to inject into the discharge a precisely known quantity of tin atoms, the minimum quantity that is required for the discharge to radiate 13.5 nm light efficiently.
The present invention advances the art by inclusion of method, apparatus and system that generates a cloud of nano-droplets for use as an X-ray, XUV, EUV, and EUV lithography light source and as a seed for a hollow cathode plasma discharge (HCPD) and dense plasma focus (DPF) source. The principle is the rapid transformation of a micro-target of mixed materials into a cloud of nano-droplets or nanoparticles. Incorporation of the nanoparticle generator into a plasma discharge light source, converts the plasma into a nanoparticle dominated plasma that produces a short-wavelength light and improves efficiency.